concat src files to destination file in order

file-concatenation.js

/**
 * Write the implementation of concatFiles(), a callback-style function that takes two or more paths
 * to text files in the filesystem and a destination file:
 * ```js
 *  function concatFiles (srcFile1, srcFile2, srcFile3, ..., dest, cb) {
 *      // ... 
 *  }
 * ```
 * This function must copy the contents of every source file into the destination file, respecting
 * the order of the files, as provided by the arguments list. For instance, given two files, if the 
 * first file contains foo and the second file contains bar, the function should write foobar (and 
 * not barfoo) in the destination file. Note that the preceding example signature is not valid
 * JavaScript syntax: you need to find a different way to handle an arbitrary number of arguments.
 * For instance, you could use the rest parameters syntax.
 */
function concatFiles(...args) {
    if (args.length <= 2) throw Error('At least one source file, one destination file, and a callback')
    const [destination, cb] = args.slice(-2)
    const srcFiles = args.slice(0, -2)
    const { readFile, appendFile } = require('fs')
    const tasks = srcFiles.map(function(file) {
        return function(callback) {
            readFile(file, function(error, data) {
                if (error) return cb(error)
                appendFile(destination, data, function(appendError) {
                    if (appendError) return cb(error)
                    callback()
                })
            })
        }
    })
    iterator(0, tasks, cb)
}
function iterator(index, tasks, finish) {
    if (index === tasks.length) return finish()
    const task = tasks[index]
    task(iterator.bind(null, index + 1, tasks, finish))
}

function callback(error) {
    if (error) return console.error(error.message)
    console.log('done')
}

concatFiles.apply(null, process.argv.slice(2).concat(['dest.txt', callback]))